The stability characteristics of helicopters are very complex and vary widely from one helicopter design to the next. Just about every individual characteristic of a helicopter affects stability in one way or another. There are certain attributes of helicopter response which are desirable for flight stability per se, and others which are desirable from the point of view of consistent response to pilot input.
It is desirable for flight stability per se to decouple the collective pitch from the longitudinal pitch axis of the helicopter. Coupling of collective pitch and longitudinal pitch axis is largely a result of locating the center of gravity aft of the center of lift. This offset causes rotations of the helicopter in its longitudinal pitch axis as a result of increases or decreases in collective pitch. In order to compensate for the coupling from collective pitch to the longitudinal pitch axis the pilot must move the cyclic stick longitudinally during collective maneuevers which increases pilot workload. An AFCS pitch channel is not normally designed to compensate for pitch perturbations of the magnitude and rate that result from collective pitch coupling.
It is desirable from the point of view of consistent response to pilot input and consistent pilot reaction to operating conditions, responses, and external perturbations to achieve positive angle of attack stability and positive speed stability, which combine to provide a desired positive relationship between longitudinal cyclic pitch stick position and airspeed (with other controls fixed), which is referred to herein as positive static pitch trim gradient. In other words, it is desirable that, with the throttle and collective pitch held constant, in maneuevering flight a rearward movement of the cyclic control is necessary to obtain a speed less than the trim speed, and a forward movement of the cyclic control is necessary to obtain a speed greater than the trim speed. As a correlary to the stability achieved by a positive static pitch trim gradient, the pilot is provided with a correct relative feel in the cyclic pitch stick; that is, the increased force, which the pilot must provide to the stick to achieve trim at increasingly forward positions, provides a relative indication of speed and/or longitudinal pitch axis inclination, on a continuous basis for any stick position, regardless of undesireable external inputs to the control system by the environment, or inadvertent pilot inputs.
An AFCS pitch channel that provides attitude hold and stability functions typically is not operable to decouple the collective and pitch axes and does not address the problem of providing positive static pitch trim gradient when it is otherwise not inherent in the handling characteristics of a helicopter. Therefore, it is known to provide a pitch bias actuator (PBA), such as an extensible link, in the longitudinal cyclic pitch channel of a helicopter with inputs as a function of airspeed multiplied inversely by collective pitch, and as a function of the rate of change of collective pitch stick position, so as to provide positive static pitch trim gradient and to decouple collective pitch from the longitudinal cyclic pitch channel at cruise airspeeds. Such a system is disclosed in U.S. Pat. No. 4,168,045 (Wright et al., 1979) entitled SPEED AND COLLECTIVE PITCH BIAS OF HELICOPTER LONGITUDINAL CYCLIC PITCH, which is incorporated by reference herein. A drawback to providing a PBA in the longitudinal cyclic pitch channel of a helicopter is the additional hardware involved, which translates into increased cost and weight and decreased reliability. Therefore it would be desirable to provide the PBA function in an existing AFCS longitudinal pitch channel. However, the PBA function requires upwards of 5% per second rate and 30% authority in order to be effective. This combination of rate and authority is not attainable in a traditional automatic flight control system (AFCS) that has a trim loop with only 2 to 3% per second rate and 100% authority, and a stability loop with 30 to 100% per second rate and only .+-.10% authority, said loops operating independently of each other. Therefore, the PBA system of the prior art uses a separate medium-rate/medium-authority actuator. In addition to the lack of compatability of the PBA function with a typical AFCS, biasing the inner loop is contrary to the null-maintaining function of the inner loop. Biasing the inner loop would reduce its available authority or completely saturate it. On the other hand, providing the PBA function in the outer loop would not "bias" the channel in the sense that a different control stick position is required for the same rotor blade pitch.
Another problem associated with providing a PBA function in an existing AFCS occurs when the AFCS is operating "single-on," or in other words with one of two redundant inner loop stability augmentation (SAS) channels shut down due to a fault. In order to prevent a hardover in the remaining SAS channel from causing an undesirable response in the aircraft, it is known to delay outer loop (trim) commands by three seconds while operating single-on so that the trim actuator, which is driven by the amplifier associated with the remaining SAS channel, does not add to the problem. In the context of providing the PBA function in the AFCS as taught hereinafter, the outer loop delay would render the PBA function virtually useless during the first three seconds of a collective maneuver in which collective coupling can cause significant responses in the pitch axis, rather quickly, thereby demanding increased pilot attention to maintaining or achieving a flight attitude.